Laser radiation may be used for processing a human eye. In a conventional laser treatment procedure, focused laser radiation is utilized to remove tissue from an exposed surface of the eye or create an incision in tissue of the eye. The removal process is oftentimes referred to as ablation in the conventional art. Whatever the physical effect (i.e. ablation or creation of an incision), a general requirement is that a focus of the radiation be steered in a precisely controlled manner in time and space so that every radiation pulse hits the eye at a desired target location.
While for the creation of an incision a patient's eye is conventionally held in a fixed position with respect to a laser apparatus delivering the radiation (through contact with a contact element of the apparatus), an ablating procedure is conventionally performed without the eye being positionally fixed. In the course of an ablating procedure, changes in eye position with respect to the laser apparatus due to a human eye's natural (and inevitable) movement or due to movement of the patient's head may thus occur. The changes in eye position may include a displacement in one or more translational directions and, alternatively or additionally, a displacement about one or more rotational axes. An eye-tracker may be used to detect eye movement and keep track of the eye's position. The eye-tracker includes one or more cameras to acquire images of the eye. Through image processing of the acquired images, a current position of the eye with respect to a coordinate system of the laser apparatus may be determined, and the determined current position may be used as a reference for aligning (centering) a shot pattern for the laser radiation. Conventionally, a position of a center of the pupil is determined as a reference position for alignment of the shot pattern. The position of the pupil center can be determined based on a detection of the margin of the pupil (i.e. iris) in the images captured by the eye-tracker. Further, it is conventionally known that the pupil center position as measured with respect to a coordinate system of the laser apparatus may shift as a result of variations of the pupil diameter. Thus, changes in ambient brightness may cause a shift of the pupil center even in the absence of eye movement.
Where an eye-tracker includes rotational tracking functionality, rotational movement of the eye can be tracked and taken into account in controlling the position of the radiation focus. For example, dynamic cyclotorsion of the eye may occur during an ablating procedure. Cyclotorsion generally refers to a rotation about an optical axis of the eye. Accordingly, whenever cyclotorsion occurs, the shot pattern should be adjusted by applying a rotational offset to account for the cyclotorsion. In addition, the optical axis may traverse the pupil at a position offset from the pupil center. A cyclotorsional movement of the eye may therefore bring with it a shift of the pupil center, requiring a translatory offset of the shot pattern to account for the pupil center shift.
The laser treatment can be a stressful experience for the patient. Nervousness of the patient typically reflects in an increased amount of natural eye movement of the patient and may also reflect in changes of the pupil diameter. A feeling of uneasiness or anxiety may also be reason for abrupt and jerking moves of the patient's eye or patient's head. If a patient is overly nervous, it may be advisable to interrupt an ongoing operation and continue at a later time after the patient has calmed down.